Causes of Respiratory Acidosis
Respiratory acidosis occurs primarily due to alveolar hypoventilation, which leads to carbon dioxide retention in the body. 1, 2
Definition and Pathophysiology
Respiratory acidosis is defined as:
- pH < 7.35 with elevated PaCO₂ (> 6.1 kPa or 46 mmHg)
- Results from the combination of CO₂ with water to form carbonic acid (H₂CO₃), which dissociates to bicarbonate (HCO₃⁻) and hydrogen ions (H⁺) 1
- Can be acute or chronic, with chronic cases showing renal compensation through bicarbonate retention 2
Major Causes of Respiratory Acidosis
1. Airway Disorders
- Chronic obstructive pulmonary disease (COPD) exacerbations 1, 2
- Severe asthma attacks 2
- Upper airway obstruction
2. Parenchymal Lung Disease
- Pulmonary edema 2
- Acute respiratory distress syndrome (ARDS)
- Severe pneumonia
- Pulmonary fibrosis
3. Neuromuscular Disorders
- Spinal cord injury 2
- Guillain-Barré syndrome
- Myasthenia gravis
- Muscular dystrophy
- Amyotrophic lateral sclerosis (ALS)
4. Central Nervous System Depression
- Drug overdose (opioids, sedatives, anesthetics) 2
- Brain injury
- Central sleep apnea
- Brainstem lesions
5. Chest Wall Abnormalities
- Severe kyphoscoliosis
- Flail chest
- Obesity hypoventilation syndrome
- Restrictive thoracic disorders 3
6. Increased CO₂ Production
- Severe hyperthermia
- Malignant hyperthermia
- Excessive carbohydrate intake in patients with limited ventilatory reserve 3
7. Increased Dead Space Ventilation
Acute vs. Chronic Respiratory Acidosis
Acute Respiratory Acidosis
- Sudden onset
- Minimal renal compensation
- Often associated with more severe symptoms
- Common in acute respiratory failure (Type II) 2
Chronic Respiratory Acidosis
- Develops over time
- Significant renal compensation with bicarbonate retention
- pH may approach normal range despite elevated PaCO₂
- Common in stable severe COPD 1
- May develop "acute on chronic" respiratory acidosis during exacerbations 1
Clinical Significance
Respiratory acidosis has significant effects on:
- Central nervous system (altered mental status, headache, confusion)
- Cardiovascular system (arrhythmias, hypotension in severe cases)
- May be accompanied by hypoxemia, especially in patients breathing room air 4
Common Pitfalls in Diagnosis
- Failing to recognize "acute on chronic" respiratory acidosis in COPD patients
- Overlooking respiratory acidosis in patients with normal pH due to renal compensation
- Not distinguishing between respiratory and metabolic causes of acidosis
- Focusing solely on oxygenation while neglecting ventilation status
Approximately 20% of patients with acute exacerbations of COPD requiring hospitalization develop respiratory acidosis, highlighting its clinical importance 1.